How do you get an ex-girlfriend back from her new boyfriend?
I think your best bet is to leave her alone. If she really does love you, then she will call and want to be with you. If she doesn't call you, then you know where you stand with her. In the meantime, go on about your life. Meet new people. Why not? Whatever you do, don't call her. Be mysterious; let her wonder what you're doing. If she does call you, act like you're having fun, and you're happy about the way your life is going.
Firstly, I need to mention that all potential solutions to your relationship issues are not a guarantee of success. Ultimately, if your ex has made a decision that your relationship is over, then you will have to make piece with it and move on.
Having said that, here are some suggestions that can help to get your girlfriend back:
1. The "No Contact Rule". This is an old strategy, but it still works. Let your ex have some space, especially if you've just broken up. They will miss the "person being around to share things with" They will miss that "person being around to share things with". Cutting off contact will also make you look stronger and less needy.
I am understanding here that you broke up with her. Well, you made the decision and if she wanted you back she would have come to you. I would leave her alone. If for some reason you don't think she is happy with her new boyfriend then perhaps you could talk to her. If she tells you she is happy with the guy she is with now I would move on.
Pretend you're not interested in her and let her know she can't have you. People always want what they can't have.
Start dating someone else immidiently to make her jealous, let her know about it. Jealousy is a powerful emotion.
Eventually tell her that your new girlfriend keeps asking for sex and you want to know if there's anything left between the two of you before you do it.
That'll spark up some emotion, trust me.